What is an example of when I can use the 'Clamp' option in the MixRGB node?
The mix node can produce results outside of the 0-1 range. For example, white divided by (almost) black gives a very high value. Take a look at this node setup:



I took 2 mix nodes, set them both to add, gave them both an input of '1' (white), and subtracted the end result with '1'. The final result of the nodes without clamping is '1' (white), while the result of the nodes with clamping is '0' (black).